Re: [RFC 2/2] block: add a fast path for seg split of large bio

[Ming Lei <ming.lei@redhat.com>]

On Thu, Jan 28, 2021 at 12:27:39PM +0000, Pavel Begunkov wrote:
> On 28/01/2021 12:10, Ming Lei wrote:
> > On Tue, Jan 05, 2021 at 07:43:38PM +0000, Pavel Begunkov wrote:
> >> blk_bio_segment_split() is very heavy, but the current fast path covers
> >> only one-segment under PAGE_SIZE bios. Add another one by estimating an
> >> upper bound of sectors a bio can contain.
> >>
> >> One restricting factor here is queue_max_segment_size(), which it
> >> compare against full iter size to not dig into bvecs. By default it's
> >> 64KB, and so for requests under 64KB, but for those falling under the
> >> conditions it's much faster.
> >>
> >> Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
> >> ---
> >>  block/blk-merge.c | 29 +++++++++++++++++++++++++----
> >>  1 file changed, 25 insertions(+), 4 deletions(-)
> >>
> >> diff --git a/block/blk-merge.c b/block/blk-merge.c
> >> index 84b9635b5d57..15d75f3ffc30 100644
> >> --- a/block/blk-merge.c
> >> +++ b/block/blk-merge.c
> >> @@ -226,12 +226,12 @@ static bool bvec_split_segs(const struct request_queue *q,
> >>  static struct bio *__blk_bio_segment_split(struct request_queue *q,
> >>  					   struct bio *bio,
> >>  					   struct bio_set *bs,
> >> -					   unsigned *segs)
> >> +					   unsigned *segs,
> >> +					   const unsigned max_sectors)
> >>  {
> >>  	struct bio_vec bv, bvprv, *bvprvp = NULL;
> >>  	struct bvec_iter iter;
> >>  	unsigned nsegs = 0, sectors = 0;
> >> -	const unsigned max_sectors = get_max_io_size(q, bio);
> >>  	const unsigned max_segs = queue_max_segments(q);
> >>  
> >>  	bio_for_each_bvec(bv, bio, iter) {
> >> @@ -295,6 +295,9 @@ static inline struct bio *blk_bio_segment_split(struct request_queue *q,
> >>  						struct bio_set *bs,
> >>  						unsigned *nr_segs)
> >>  {
> >> +	unsigned int max_sectors, q_max_sectors;
> >> +	unsigned int bio_segs = bio->bi_vcnt;
> >> +
> >>  	/*
> >>  	 * All drivers must accept single-segments bios that are <=
> >>  	 * PAGE_SIZE.  This is a quick and dirty check that relies on
> >> @@ -303,14 +306,32 @@ static inline struct bio *blk_bio_segment_split(struct request_queue *q,
> >>  	 * are cloned, but compared to the performance impact of cloned
> >>  	 * bios themselves the loop below doesn't matter anyway.
> >>  	 */
> >> -	if (!q->limits.chunk_sectors && bio->bi_vcnt == 1 &&
> >> +	if (!q->limits.chunk_sectors && bio_segs == 1 &&
> >>  	    (bio->bi_io_vec[0].bv_len +
> >>  	     bio->bi_io_vec[0].bv_offset) <= PAGE_SIZE) {
> >>  		*nr_segs = 1;
> >>  		return NULL;
> >>  	}
> >>  
> >> -	return __blk_bio_segment_split(q, bio, bs, nr_segs);
> >> +	q_max_sectors = get_max_io_size(q, bio);
> >> +	if (!queue_virt_boundary(q) && bio_segs < queue_max_segments(q) &&
> >> +	    bio->bi_iter.bi_size <= queue_max_segment_size(q)) {
> > 
> > .bi_vcnt is 0 for fast cloned bio, so the above check may become true
> > when real nr_segment is > queue_max_segments(), especially in case that
> > max segments limit is small and segment size is big.
> 
> I guess we can skip the fast path for them (i.e. bi_vcnt == 0)

But bi_vcnt can't represent real segment number, which can be bigger or
less than .bi_vcnt.

> I'm curious, why it's 0 but not the real number?

fast-cloned bio shares bvec table of original bio, so it doesn't have
.bi_vcnt.


-- 
Ming